To provide a composition for a thermosetting silicone resin having excellent light transmittance, adhesiveness, light resistance and heat resistance and high mechanical strengths, a thermosetting silicone resin composition produced by reacting the composition, a method for producing the resin composition, an encapsulating material for an optical semiconductor element containing the thermosetting silicone resin composition, and an optical semiconductor device encapsulated with the resin composition or the encapsulating material for the optical semiconductor element.

The composition for the thermosetting silicone resin contains (A) an organohydrogenpolysiloxane, (B) an epoxy compound containing an alkenyl group, (C) a cyclic siloxane containing an alkenyl group and (D) a hydrosilylation catalyst.

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